1. Field of the Invention
The present invention relates to a method and apparatus for controlling transmission functions in a transmission of a motor vehicle by means of an electromagnetically actuated control valve. The control valve is actuated by means of a temperature-dependent and target-current-dependent overlay, so that a symmetrical current value oscillation around the particular target current value is achieved.
2. Description of the Related Art
A method and apparatus for controlling transmission functions in a transmission of a motor vehicle are adequately known from existing motor vehicle technology. Generally a proportional valve is used as the control valve, to actuate a clutch pressure, a shifting pressure in the transmission, or a cooling flow, for example. To prevent so-called stick-slip effects and to reduce hysteresis, the proportional valve is actuated with a temperature-dependent and target-current-dependent overlay. The overlay is a symmetrical current value oscillation around the target current value. If the characteristic of the proportional valve is linear, those overlays also result in symmetrical pressure oscillations around the target pressure.
Because of the non-linearity of the characteristic curve of the proportional valve, a shift of the mean pressure comes about in the non-linear region, since the non-linearity brings about a change in the rise of the characteristic curve of the proportional valve. For example, when there is a positive change in the rise of the characteristic curve, an increase in the current value also brings about an increase in the pressure value. On the other hand, a reduction of the current value brings about only a small reduction of the pressure in this non-linear region. That causes a shift in the mean pressure value. In addition, the temperature-dependency of the overlay, which increases the overlay amplitude at low temperatures because of the elevated oil viscosity, can further amplify the shift in the mean pressure.
Accordingly, an object of the present invention is to provide a method and apparatus of the type identified at the beginning, in which a shift of the mean pressure is also prevented in the non-linear region of the characteristic curves of the control valve.